A crucial element in the development of effective vaccine strategies for AIDS is an experimental animal model in which the course of immunodeficiency virus infection parallels the pathogenesis of the human disease. SIV infection of macaques is a relevant model since it induces an immunodeficiency syndrome in infected macaques that is remarkably similar to human AIDS. VACCINE STUDIES: Our prior vaccine studies focused on the use of the highly attenuated modified vaccinia virus Ankara to express SIV proteins. Immunization with MVA-SIV recombinants resulted in a reduction in setpoint plasma viral load that was associated with prolonged survival. Better preservation of memory CD4+ T cells was correlated with titers of neutralizing antibodies to the challenge virus prior to challenge suggesting an important role for NAb and Envelope immunogens in this model. We therefore used a prime-boost, envelope-based approach as a strategy to generate neutralizing antibody responses. For this strategy we used a MVA recombinant expressing the neutralization-resistant SIVsmE543-3 envelope as the prime and trimeric SIVsmE543-3 envelope generated from a plasmid of codon-optimized gp140 as a boost. Rhesus macaques were immunized with either: A) Trimer alone, B) MVA-nonrecombinant, or C) MVA-Env prime, + trimer boost; all were given with adjuvant (AbISCO-100). All SIV Env immunized animals developed neutralizing antibodies to Tier 1 SIVsmE660 but only weak and partial neutralization of Tier 3 SIVsmE543-3 immunogen. They were challenged intrarectally with a single dose of SIVsmE660, 4 months following the last immunization. Four of six macaques that received prime-boost showed a 1-2 weeks delay of peak viremia accompanied with one log reduction in both peak and set point viremia when compared to control macaques vaccinated with non-recombinant MVA. The plasma viral load in these 4 macaques has been below detection limits from 18 weeks post challenge through 2-3 year of follow-up. Macaques vaccinated only with purified Env trimer uniformly showed earlier peak of viremia that was even higher than viral load shown by animal in control group. Despite this apparent enhanced acute viremia, two macaques from this group also were able to control viremia after an acute phase of infection. Control of viremia in these monkeys was associated with higher degree of preservation both central and effector memory subsets of CD4+ T cells and increased activation of B cells, as compared to non-controllers. Studies are underway to evaluate whether the breadth of neutralizing antibody responses in these animals correlates with control of viremia. NEUTRALIZING ANTIBODY RESPONSES AFTER SIV-INFECTION: Another focus of the lab has been the characterization of the breadth of neutralizing antibody responses after challenge with SIVsm. For these studies, we generated full length infectious clones of the challenge stock, SIVsmE660 by RT-PCR from the virus stock. Three of these viruses replicate efficiently in rhesus PBMC in vitro and were very sensitive to neutralizing antibody (equivalent to Tier 1 classification used for HIV-1. Two clones were evaluated for in vivo viral replication and pathogenesis rhesus macaques following intravenous inoculation. Both clones resulted in robust and persistent viremia that was associated with significant loss of CD4+ memory T cells. Evolution of the V1 and V4 regions was associated with escape from neutralization. We developed a panel of chimeric E660 clones expressing a range of late stage envelopes with varying sensitivity to neutralization cloned from plasma of these animals. Screening of these chimeric clones using post challenge sera from SIVsmE660-infected macaques allowed us to define at least three groups roughly equivalent to the Tiers 1, 2 and 3 described for HIV-1. We created and characterized representative Tier 1, Tier2 and Tier 3 variants from this panel and are currently evaluating their infectivity, viral replication and associated pathogenesis in rhesus macaques. These studies have used animals expressing the moderately susceptible TRIMTFP/Q genotype to avoid TRIM5 restriction as a confounding factor (see AI000370). The goal of this study is to determine whether the neutralization sensitivity of the infecting SIV influences viremia, generation of autologous and heterologous neutralizing antibody and/or disease progression. POST EXPOSURE NEUTRALIZING ANTIBODYTHERAPY. Prevention of mother-to-child transmission (MTCT) of HIV remains a major objective where antenatal care is not readily accessible. We tested HIV-1-specific human neutralizing monoclonal antibodies (NmAbs) as a post-exposure therapy in an infant macaque model for intrapartum MTCT. One-month-old rhesus macaques were inoculated orally with the simian-human immunodeficiency virus SHIVSF162P3. On days 1, 4, 7 and 10 after virus exposure, we injected animals subcutaneously with NmAbs and quantified systemic distribution of NmAbs in multiple tissues within 24 h after antibody administration. Replicating virus was found in multiple tissues by day 1 in animals that were not treated. All NmAb-treated macaques were free of virus in blood and tissues at 6 months after exposure. We detected no anti-SHIV T cell responses in blood or tissues at necropsy, and no virus emerged after CD8(+) T cell depletion. These results suggest that early passive immunotherapy can eliminate early viral foci and thereby prevent the establishment of viral reservoirs.